Patent Literature 1 discloses a technique of reusing an oil discharged from an actuator (a technique of recycling a pressure oil). In the technique disclosed in this literature, the amount of charge (the discharge rate) of each of two pumps (12L, 12R) is individually controlled by the negative control. More specifically, claim 1 of the above literature recites as follows. “The pressure oil flowing out from a bottom-side oil chamber of a boom cylinder is allowed to flow into another hydraulic actuator, and the discharge rate reduction unit reduces the discharge rates of the main pumps.” The above literature also discloses in paragraph 0019 as follows. “The flow of the pressure oil discharged from the main pumps (12L, 12R) is restricted by the negative control throttles (20L, 20R), . . . the negative control throttles (20L, 20R) produce a control pressure (hereinafter referred to as “a negative control pressure”) for controlling regulators (13L, 13R).” The above literature also discloses in paragraph 0021 as follows. “The regulators (13L, 13R) reduce the discharge rates of the main pumps (12L, 12R) as the negative control pressure introduced thereto is larger, and these regulators increase the discharge rates of the main pumps (12L, 12R) as the negative control pressure introduced thereto is smaller.” The reference signs in the above literature are enclosed within parentheses.